Flow conditioner

ABSTRACT

A flow conditioner comprising an apertured circular plate to be placed in a conduit in an orientation substantially perpendicular to the axis of the conduit. The apertures are circular and are arranged in a plurality of radially spaced circular arrays around a central aperture. The center of the central aperture and the centers of the circular arrays coincide with the center of the circular plate. The apertures in each circular array are equally spaced apart around the center of the plate, and all the apertures in any one circular array are of substantially the same diameter. The size and number of apertures are such that the impedance to flow presented by the plate increases with the radius on which a given array of apertures is arranged.

The present invention relates to flow conditioners.

The International Standard on the Measurement of Fluid Flow by Means ofOrifice Plates (ISO 5167) states that acceptable flow conditions formetering purposes will be attained when at each point across a pipecross-section the ratio of local axial velocity to maximum axialvelocity at the cross-section agrees to within ±5% with that which wouldbe attained in swirl-free flow at the same radial position at across-section located towards the end of a long length of straight pipe,that is a pipe the length of which exceeds 100 pipe diameters. Swirlfree conditions are considered to exist when the swirl angle over thepipe cross-section is less than 2°.

Flow non-uniformities, affecting both mean flow, turbulence structureand swirl angle, make it extremely difficult to guarantee this standardof flow quality. Non-uniformities are introduced by bends, valves andother pipe fittings. Straight lengths of pipe of well over 100 pipediameters in length would be necessary upstream of the metering point toachieve satisfactory swirl values and there is experimental evidence tosuggest that even for mean flow levels such a length would not besufficient.

Clearly to provide such a settling length, even if practical given theconstraints of the pipe layout, requires a large capital outlay forpipework. Particularly for large diameter pipes this may outweigh thecommercial returns to be gained from the metering project to beundertaken, even possibly making it non-viable. To reduce the settlinglength necessary to achieve an adequately uniform flow, it is known tointroduce a flow conditioning device which is designed to produce thenecessary flow conditions within a relatively short length of pipe.Three types of flow conditioner are generally considered, that is a tubebundle flow straightener, a Sprenkle flow straightener, and a Zankerflow straightener. These devices details of which can be obtained fromthe above-mentioned ISO Standard, partially block the pipe into whichthey are introduced, resulting in a significant loss in pressure, Δ p,usually represented in terms of a loss coefficient, K=Δp/(1/2pu²)₁.Typical K values for the three conditioners specified above are 5, 15,and 5, respectively.

Even with the inclusion of such a flow conditioner it is recommendedthat 22 pipe diameters should be allowed downstream of the conditionerbefore the meter and that at least 20 straight pipe diameters betweenthe pipe fitting producing the flow non-uniformity and the conditionershould be provided. Thus even with the above conditioning devices atleast 42 pipe diameters is needed in the metering length.

A flow conditioner is described in British Patent Specification No.1375908 which has considerable advantages over the devices previouslymentioned. These advantages are:

(i) Simplicity of manufacture. The described device is essentially aperforated plate consisting of 35 holes of the same diameter. Each holehas a diameter equal to 0.13× the pipe diameter.

(ii) Low pressure loss coefficient K of approximately 1.4.

(iii) Relatively short downstream length necessary to producesatisfactory flow conditions.

(iv) Easy to install between pipe flanges as unit depth is 0.13× pipediameter, (that is the same as the hole size).

(v) Easily adapted to different pipe sizes--unit scales with pipediameter.

Despite these advantages the device described in British PatentSpecification No. 1375908 is not ideal. Specifically the holedistribution is not axi-symmetric and therefore its effect on flowconditions is, at least in the early stages, sensitive to theorientation of the flow conditioner to the flow. Furthermore, althoughthe low pressure loss coefficient is advantageous where pressure lossesmust be minimized, the attenuation of upstream flow non-uniformities islimited.

It is an object of the present invention to provide an improved flowconditioner.

According to the present invention there is provided a flow conditionercomprising an apertured circular plate intended to be placed in aconduit in an orientation substantially perpendicular to the axis of theconduit, wherein the apertures are circular and are arranged in aplurality of radially spaced circular arrays around a central aperture,the centre of central aperture and the centres of the circular arrayscoincide with the centre of the circular plate, the apertures in eachcircular array are equally spaced apart around the centre of the plate,all the apertures in any one circular array are of substantially thesame diameter, and the size and number of apertures in the circulararrays are such that the impedance to flow presented by the plateincreases with the radius on which a given array of apertures isarranged.

Preferably, the conditioner comprises an inner array of n aperturesadjacent to the central aperture and an outer array of m aperturesadjacent to the inner array, the plate having a diameter D, the centresof the apertures of the inner and outer arrays being located on circlesof diameter D₁ and D₂ respectively, the central aperture having adiameter d₁, the apertures of the inner and outer arrays havingdiameters d₂ and d₃ respectively, and the array radii and aperturediameters being related in accordance with:

    nd.sub.2 D.sub.2 >md.sub.3 D.sub.1

Such an arrangement ensures that the impedance to flow increases withradius. This simulates normal developed flow conditions in which thevelocity in a pipe reduces towards the pipe wall.

Preferably the diameter of the central aperture is greater than thediameter of the apertures in the circular arrays, and for any adjacentpair of circular arrays, the apertures in the radially inner array ofthe pair are of greater diameter than the apertures in the radiallyouter array of the pair. The open area of the plate corresponding to thesum of the areas of the apertures is preferably from 50 to 60 percent ofthe total area of the plate. The pressure loss coefficient may be atleast 2.7, and the plate thickness may be at least twelve percent of theplate diameter.

Preferably the upstream edges of each aperture are chamfered.

Various aperture arrangements may be used, e.g. a 1-7-13 arrangement, ora 1-5-12 (See FIG. 9), a 1-7-12 (See FIG. 7), a 1-6-14 (See FIG. 8) or a1-7-11 (see FIG. 6) arrangement.

The arrangement of apertures in accordance with the invention leads to aplate with a variable resistance coefficient producing a downstream flowwhich quickly develops to approach fully developed flow conditions.

The advantages of the present invention as compared with the devicedescribed in British Patent Specification No. 1375908 are as follows:

(i) A greater degree of axial symmetry in the downstream flow close tothe plate, contributing to more rapid flow mixing.

(ii) Superior ability to deal with certain flow distortions.

(iii) Downstream swirl values reduced with greater certainty.

(iv) Easier to manufacture and specify.

Even though the K value is preferably higher than for that described inBritish Patent Specification No. 1375908 it is relatively low comparedto other flow conditioning devices in common usage where K values of 5to 6 are typical and values of well over 15 are tolerated in somecircumstances.

An embodiment of the present invention will now be described, withreference to the accompanying drawings, in which:

FIG. 1 is a front view of a flow conditioning plate of the typedescribed in British Patent Specification No. 1375908;

FIG. 2 is a section on the line II--II of FIG. 1;

FIG. 3 is a front view of a flow conditioning plate in accordance withthe present invention;

FIG. 4 is a sectional view on lines IV--IV of FIG. 3, and

FIG. 5 illustrates an alternative configuration in which the upstreamedges of the apertures are chamfered

FIG. 6 is an alternate embodiment of the flow conditioning plate of theinvention.

FIG. 7 is another alternate embodiment of the flow conditioning plate ofthe invention.

FIG. 8 is still another alternate embodiment of the flow conditioningplate of the invention.

FIG. 9 is yet another alternate embodiment of the flow conditioningplate of the invention.

Referring to FIGS. 1 and 2, the illustrated flow conditioning platecomprises thirty five apertures 1 each having a diameter equal to 0.13times the diameter of the plate in which they are formed. The thicknessof the plate is also equal to 0.13 times the diameter. Essentially thepattern of apertures is arranged such that for the given aperturediameter the maximum number of apertures is provided. This means thatthere is a central aperture, a first array of six apertures therearound,a second array of twelve apertures arranged around the outside of thefirst array, and a further array of sixteen apertures distributed aroundthe periphery of the plate. Such an arrangement provides improvedperformance compared with traditional flow conditioning devices, forexample the bundle flow straightener, the Sprenkle flow straightener andthe Zanker flow straightner.

Referring now to FIGS. 3 and 4, the illustrated embodiment of theinvention comprises a circular plate in which an axi-symmetrical arrayof apertures has been formed. The apertures comprise a central aperture2 of diameter d₁, a first intermediate ring of seven apertures 3 ofdiameter d₂, and an outer ring of thirteen apertures 4 of diameter d₃.All of the apertures are circular and the centre of each aperture 3 ispositioned on an imaginary circle 5 of diameter D₁ centered on thecentre of plate. The centres of all the apertures 4 are positioned on animaginary circle 6 of diameter D₂ also centered on the centre of theplate.

The diameters of the various apertures may be specified in terms of theplate diameter D as follows:

The central aperture 2 has a diameter of 0.192 D, the intermediateapertures 3 have diameters of 0.1693 D, and the outer apertures 4 havediameters of 0.1462 D. The angular spacing between adjacent apertures 4corresponds to the angles subtended between lines 7 and 8 and is equalto 27.69 degrees. The angular spacing between the apertures 3corresponds to the spacing between lines 9 and 10 which subtend an angleof 51.428 degrees. The diameter of the circle 5 is equal to 0.46158 Dand the diameter of the circle 6 is equal to 0.8436 D. The thickness ofthe plate is equal to 0.123 D. The plate diameter D corresponds to theinternal diameter of the pipe in which it is positioned. It will beappreciated therefore that the plate will normally incorporate a flange(not shown) to enable it to be fitted in the pipe conveniently.

Thus it can be seen from FIG. 3 and FIG. 4 that the arrangement ofapertures is axi-symmetric radially. The radially inner edges of theapertures 4 touch an imaginary circle which is radially outside theradially outer edges of the apertures 3, and the aperture sizes decreasein a radially outward direction. This arrangement makes it possible toadjust the percentage of the pipe surface area which is occupied byapertures to achieve a desired pressure loss coefficient. In theillustrated case the pressure loss coefficient is 2.7.

As illustrated in FIG. 5, the upstream edges of the apertures may bechamfered. In the illustrated case, the chamfered edges subtend an angleof 45° to the plate surface. The depth of the chamfered edges may beequal to D/64.

It will be appreciated that embodiments of the present invention can beproduced in which the numbers of apertures in the circular arrays ofapertures differs from that shown. In the illustrated case the aperturesare arranged in a 1-7-13 pattern. Other patterns would be possiblehowever, for example 1-7-12 (See FIG 7), 1-6-14 (See FIG. 8) etc. Thusit is not the number of apertures in each circular array that is ofprime importance but rather the general structure of the pattern and theproportion of the plate occupied by the apertures. In the illustratedcase, the overall open area or porosity (that is the proportion of thearea occupied by apertures) is between 51 and 52%. A preferred range forthe overall open area is 50 to 60%.

A series of tests have been conducted to compare the performance of theflow conditioners illustrated in FIGS. 1 and 3. The tests were conductedin pipe rigs of 0.10325 m diameter at a mean velocity of 28 m/sequivalent to a Reynolds number based on mean velocity and pipe diameterof approximately 2×10⁵. Some of the test conditions were produced in along closed circuit pipe rig where sufficient development length toproduce essentially fully developed flow conditions was available. Othertests were carried out in shorter open circuit facilities which allowtest layouts to be more easily varied. However irrespective of the testfacility used all the measurements were made in similar test sectionsand with identical instrumentation.

The test conditions which have been simulated upstream of theconditioner include:

Test A: Fully developed flow condition

Test B: Non-uniform flow conditions produced by a partly closed ballvalve

(i) Setting 1 (approximately 1/4 closed)

(ii) Setting 2 (approximately 1/2 closed)

(iii) Setting 3 (approximately 3/4 closed)

Test C: Axi-symmetric flow conditions

(i) Uniform upstream flow.

(ii) Highly peaked upstream flow.

(iii) Wake upstream flow.

Test D: Non-uniform flow condition produced downstream of two 90° bendstaking the flow out of the initial approach plane.

Test A--Fully Developed Upstream Flow

Test A was carried out in a long closed circuit test facility in whichsufficient length to achieve fully developed conditions was available. Aflow conditioner should not disturb an already fully developed flow (orshould be able to return the flow to fully developed within thestipulated settling length). Not only should it restore the mean flowprofile to its initial state it should also leave the turbulencestructure unchanged. There is documentary evidence to suggest that someearlier flow conditioners rather than improve the performance ofdownstream flow meters can in effect make things worse when the upstreamflow is already fully developed because of their inability to cope withfully developed upstream conditions.

The test arrangement was such that the flow conditioner was placed 1.5 Ddownstream of a traverse plane which was located 102 pipe diameters fromthe contraction outlet plane of a long straight pipe. Mean flow andaxial turbulence intensity readings were made at stations 2.5, 5.5 and8.5 pipe diameters downstream of the conditioner and compared with theupstream profile measured 1.5 diameters upstream of the conditioner.

In the case of the FIG. 3 unit, the mean flow was close to the upstreamflow within a pipe length of 5.5 D. The profile at 8.5 D showed nochange from the profile at 5.5 D and was symmetrical about the pipeaxis. In the case of the FIG. 1 unit the centre core of the flow wasaccelerated substantially at 2.5 D. Although at 8.5 D the centre corehad decayed to approach the upstream speed level, there was asignificant difference between upstream and downstream flows.Furthermore, the turbulence level at the centre of the flow was reduced,and the flow profile was asymmetric. Ideally a conditioner shouldproduce fully developed flow conditions downstream irrespective of theupstream flow quality. The turbulence structure as well as the time meanflow associated with fully developed flow should be reproduced. Thusclearly for test A the FIG. 3 unit performed better than the FIG. 1unit.

Test B--Upstream Condition Produced by Partially Closed Ball Valve

A series of tests were carried out in a short open circuit test rig inwhich a ball valve was installed downstream of the tunnel contractionand upstream of the conditioning unit.

The conditioner was placed 3 pipe diameters from the valve outlet planeand measurements were made 1.5 pipe diameter upstream of the conditionerand at stations 1.5, 4.5, 8.5 and 10.5 pipe diameters downstream of theconditioner. Because of the inherent asymmetry in the flow produced bythe valve measurements were made in two diametric planes turned through90° (labelled arbitrarily 0° and 90° ) so that the degree of asymmetryin the flow downstream of the conditioning unit could be assessed.

Measurements of mean flow and table of swirl values were obtained forthree different cases corresponding to different valve closure settings.

Case B(i) valve approximately 1/4 closed

With the unit of FIG. 3, the upstream flow was highly distorted but thedownstream flow was within acceptable limit within 4.5 diameters of theconditioner for both the 0° and 90° cases. The maximum swirl anglemeasured at 4.5 diameters was 1° . With the unit of FIG. 1, the upstreamprofile distortion was similar to that for the FIG. 3 unit. However thedownstream profiles at 0° was just within acceptable limits at 8.5 and10.5 D but for the 90° case there was a detectable portion of the flowwhich was outside acceptable limits even at 10.5 D. The maximum swirlangle at 10.5 D was 2° . Thus, the performance of the FIG. 3 unit wasclearly superior.

Case B(ii) Valve Approximately 1/2 close

The upstream flow profiles were much more severely distorted withportions of the flow in which flow separation and reversal has occurred.

In the case of the FIG. 3 unit, for both the 0° and 90° cases thedownstream flow was within acceptable limits at 8.5 D and at 8.5 D themaximum swirl angle was 1° . In the case of the FIG. 1, for the 0° setof results the downstream flow was within acceptable limits at 8.5 Dwhereas at 90° there was a portion of the downstream flow which even at10.5 D was outside acceptable limits. Swirl valves of 3° at 10.5 D werenoted. Thus again the FIG. 3 unit performance was superior to that ofthe FIG. 1 unit.

Case B(iii) Valve Approximately 3/4 closed

The upstream profile was even more distorted with large regions ofseparation and reversal. In the case of the FIG. 3 unit, for the 0° and90° cases the downstream profile was within acceptable limits at 10.5 Dand the swirl values at that station were 0° . In the case of the FIG. 1unit, although the profiles for the 0° case were within acceptablelimits at 8.5 D they were outside acceptable limits at 90° even at 10.5D. Maximum swirl values of 2° were noted at 10.5 D. Therefore the FIG. 3unit again outperformed the FIG. 1 unit.

Test C--Axi-symmetric Test Conditions

These series of tests were again carried out in an open circuit test rigin which different forms of resistance were introduced to produceartificially distorted flow conditions upstream of the conditioner. Forcase (i) i.e. uniform approach flow the upstream condition was a profileproduced close to the tunnel contraction outlet plane where the wallboundary layer was thin. For cases (ii) and (iii) a flow generator wasintroduced into the flow giving for case (ii) a peaked upstream flow(centre velocity greater than mean velocity) and for case (iii) a wakeflow (centre velocity less than mean velocity). Traverses were made atstations 1 diameter upstream of the conditioner and 4.5, and 8.5 and10.5 diameters downstream of the conditioner.

Case (i) Uniform Approach Flow

In the case of the FIG. 3 unit, the flow was within acceptable limits at4.5 D. In the case of the FIG. 1 unit, the profiles downstream havemaximum velocity levels of about 1.28 times mean velocity which withinthe test range showed no signs of decaying. Thus even at 10.5 D from theconditioner the profiles were outside acceptable limits.

Case (ii) Peaked Upstream Flow

The generator used to produce a peaked flow condition was an orificeplate with a central hole of diameter D/2. The conditioner was placed4.5 D from the generator. A highly distorted flow was produceddownstream of the orifice plate which in the development length provideddecayed to give a peaked flow more typical of a profile found inindustrial situations. The resulting upstream flow is peaked with amaximum velocity of about 1.5 times mean velocity. The upstream flowcondition was measured 1.5 diameters upstream of the conditioner and thedownstream profiles measured at 2.5, 4.5 and 8.5 diameters downstream ofthe conditioner. For both the FIG. 1 and FIG. 3 units, the downstreamflow profiles were within acceptable limits at 4.5 D.

Case (iii) Wake upstream flow

The wake flow was generated by blocking the central portion of the flowwith a solid disc supported by thin strips at the periphery of the pipe.The ratio of disc diameter to pipe diameter was 0.5. Again 4.5 pipediameters of development length was allowed between the generator andthe conditioner and the upstream profile was measured 1.5 diametersupstream of the conditioner. The upstream profile produced was highlydistorted. In the case of the FIG. 3 unit, the downstream profiles werewithin acceptable limits as close as 1.5 diameters to the conditioner.In the case of the FIG. 1 unit, the downstream profiles were justoutside acceptable limits at 1.5 and 4.5 diameters but inside the limitsfrom 8.5 diameters. Thus for test C for the FIG. 3 unit either exceededor matched the performance of the FIG. 1 embodiment.

Test D

These tests were carried out in an open circuit test rig connected totwo right angled bends taking the flow out of the approach plane. Theconditioner was placed 3 diameters from the outlet flange of the secondbend and measurements were made1.5 diameters upstream of the conditionerand 2.5, 5.5 and 8.5 diameters downstream of the conditioner in twoorthogonal diametric planes (0° and 90°). Because of the ductingarrangement it was only possible to measure the upstream profile for the0° setting.

For the FIG. 3 unit, in the 0° case the downstream profiles were withinacceptable limits at 5.5 ° D. For the 90° case the profiles were withinacceptable limits at 8.5 D . The maximum swirl angle at 8.5 D was 1° .For the FIG. 1 unit, the downstream profiles for both the 0° and 90°cases were within acceptable limits at 4.5 D but the swirl anglesalthough not exceeding a maximum magnitude of 2° varied between -2° and+2° at 8.5 D . Thus even in this test the FIG. 3 unit gave betterresults because of the significantly lower downstream swirl variationproduced.

To summarize the test results, for most of the upstream test cases theFIG. 3 unit performed better than the FIG. 1 unit. For all the testcases apart from case B(iii), which represented the most severe upstreamdistortion setting the FIG. 3 unit produced a downstream flow which waswithin acceptable limits within 8.5 diameters of the conditioner withcorresponding swirl angles with a maximum value of 1° . The conditionerwas placed 3 pipe diameters downstream of the source of disturbance(e.g. three diameters from the outlet plane of a bend or partiallyclosed valve). Thus the FIG. 3 unit would require a maximum settlinglength of 12 pipe diameters between source of disturbance and flow meterwith 3 diameters between source and conditioner and a 9 diametersettling length downstream. For many test conditions an even shorterdownstream length would be acceptable. This compares with a claimedlength of eleven diameters for the FIG. 1 unit, but as is clear from theabove there is evidence to show that in many instances this unit doesnot give an acceptable flow quality given the specified downstreamsettling length.

Versions of the FIG. 3 unit have been tested in pipes of 140 mm and 312mm diameter where the conditioning unit has been scaled with the pipesize. These results confirmed the acceptable flow quality produced bythe FIG. 3 unit.

Mention is made above of the fact that hole arrangements other than the1-7-13 arrangement of FIG. 3 are possible. Various alternative holearrangements are discussed below.

A perforated plate could be produced with a central hole, n holes on ap.c.d (pitch circle diameter) of 0.4616 D, where D is the platediameter, and an outer ring of m holes on a p.c.d of 0.8436 D, If allthe holes were of the same size, diameter d, then in order to make thetotal open area the same as the 1-7-13 plate:

    (d/D).sup.2 +n(d/D).sup.2 +m(d/D).sup.2 =0.5156

i.e. if N=n+m+1 is the total number of holes

    d/D=0.718/√N

To maintain the same open area in each of the two outer rings i.e. thesame porosity grading, it is necessary that:

    n(d/D).sup.2 =0.2006

and

    m(d/D).sup.2 =0.27786

i.e. n/m=0.722

The hole diameter must be such that the inner and outer rings of holesdo not overlap and that the outer ring of holes lie within the pipediameter D. This means that d/D<0.1564. i.e. d/D=0.718/√N<0.1564 i.e.N>22

Also to ensure that the two arrays of holes can be accommodated withinthe two selected pitch circles it is necessary that nd<1.45 D andmd<2.65 D.

Looking at what this now implies for a particular plate the followingtable applies:

    ______________________________________                                                                             nd/   md/                                N   d/D     n         m        n/m   1.45D 2.65D                              ______________________________________                                        22  0.15308  8.56 = 9 11.85 = 12                                                                             0.75  0.9502                                                                              0.693                              23  0.1497   8.49 = 9 12.39 = 13                                                                             0.692 0.929 0.734                              24  0.14656  9.338 =  12.936 = 13                                                                            0.769 1.01  0.718                                          10                                                                25  0.1436   9.728 =  13.475 = 14                                                                            0.714 0.99  0.7586                                         10                                                                26  0.1408  10.12 = 11                                                                              14.014 = 14                                                                            0.7857                                                                              1.068 0.7438                             ______________________________________                                    

i.e. only for N=22,23 and 25 can the middle array be accommodated andeven for these values of N the holes would have practically run intoeach other.

Even for these values of N it is doubtful whether the plate couldperform as well as the original 1-7-13 plate since the open area of thecentral hole would be considerably reduced. It is unlikely thereforethat a plate of uniform aperture size could be preferable to the unitshown in FIG. 3. Nevertheless useful results could be obtained with auniform aperture size.

Having discussed possible plate configurations in which the holes areall of the same size, arrangements with varying hole sizes are discussedbelow.

Assuming a perforated plate with a central hole of diameter d1, n holesof diameter d2 on a p.c.d. of 0.46158 D and an outer ring of m holes ofdiameter d3 on a p.c.d. of 0.8436 D, then in order to make the totalopen area the same as the original 1-7-13 plate it is necessary for:

    (d1/D).sup.2 +n(d2/D).sup.2 +m(d3/D).sup.2 =0.5156

To maintain the same open area in each of the two outer rings i.e. thesame porosity grading, it is necessary that:

    n(d2/D).sup.2 =0.2006

and

    m(d3/D).sup.2 =0.27786

assuming d1=0.1924 D as in the original 1-7-13 plate.

Again it is necessary to ensure that the hole diameter is such that theinner and outer rings of holes do not overlap and that the outer ring ofholes lie within the pipe diameter D. This means that the followingconditions must apply:

    d2<0.26918 D d2+d3<0.382 D and d3<0.1564 D

As the two arrays of holes must be accommodated within the two selectedpitch circles we must have nd2/D<1.45 and md3/D<2.65.

For such a plate the following table applies:

    ______________________________________                                        OUTER RING OF HOLES                                                                            INNER RING OF HOLES                                                              gap                     gap                                           md.sub.3 /                                                                            between           nd.sub.2 /                                                                          between                           m   d.sub.3 /D                                                                            D/2.65  holes/D                                                                              n   d.sub.2 /D                                                                           D/1.45                                                                              holes/D                           ______________________________________                                         3  0.3043  d.sub.3 >       3  0.25858                                                    .1564                                                              4  0.2636  d.sub.3 >       4  0.22394                                                    .1564                                                              5  0.2357  d.sub.3 >       5  0.200299                                                                             0.6907                                                                              0.09                                          .1564                                                              6  0.2152  d.sub.3 >       6  0.1828 0.7564                                                                              0.06                                          .1564                                                              7  0.1993  d.sub.3 >       7  0.16923                                                                              0.817 0.04                                          .1564                                                              8  0.1864  d.sub.3 >       8  0.1583 0.8733                                                                              0.02                                          .1564                                                              9  0.1757  d.sub.3 >       9  0.1493 0.9267                                                                              0.01                                          .1564                                                             10  0.1667  d.sub.3 >      10  0.1416 0.97655                                                                             0.003                                         .1564                                                             11  0.1589  d.sub.3  >                                                                    .1564                                                             12  0.1522  0.687   0.069                                                     13  0.1462  0.717   0.057  11  0.13504                                                                              >1                                      14  0.1409  0.744   0.048  12  0.1293 >1                                      15  0.13601 0.77    0.041  13  0.1242 >1                                      16  0.1318  0.796   0.033  14  0.1197 >1                                      17  0.12786 0.82    0.028  15  0.1156 >1                                      18  0.12426 0.84    0.023  16  0.11197                                                                              >1                                      19  0.12093 0.8675  0.018  17  0.10868                                                                              >1                                      20  0.1179  0.889   0.015  18  0.10556                                                                              >1                                      21  0.11503 0.91    0.011                                                     22  0.11238 0.933   0.008                                                     23  0.1099  0.954   0.005                                                     24  0.10759 0.97    0.0028                                                    25  0.1054  0.994   0.0006                                                    26  0.10338 >1                                                                ______________________________________                                    

Thus taking into account the constraints on d2 and d3 then 3<n<10 and12<m<25. For n=3,m must be >19 or d2+d3 >0.382 D. Also it is worthnoting that from a structural point of view the higher values of m and nwill leave the plate fairly weak and difficult to manufacture.

The last column in the table indicates that for m>19 less than 0.002 Dof material is left between adjacent holes, which for D<100 mm impliesless than 2 mm of material. Also for n=3 very little material would beleft between the two rings of holes since d2+d3 will be >0.382 when m<19or close to 0.382 for m>19.

It would appear that the plate with the minimum number of holes is1:5:12 (See FIG. 9) although with this arrangement there may be littlematerial left at some points between the two rings of holes.

A safer combination could be to go to the next arrangement i.e. 1:6:12in which it would be possible to achieve almost complete axial symmetryin the hole arrangement.

It is possible to specify the arrangement of apertures by reference tothe porosity P, where P is the ratio of the open area of the plate tothe total plate area. In the embodiment described with reference to FIG.3, P=0.5155. For a plate defined as above with a central aperture ofdiameter d₁, an inner ring of apertures of diameter d₂, and an outerring of apertures of diameter d₃, the inner ring having a pitch circlediameter of 0.4616 D, and the other ring having a pitch diameter of0.8436 D, then: ##EQU1##

In order to accommodate the rings of apertures on the specified pitchcircles (different pitch circle diameter could of course be specified),and to ensure that the apertures do not overlap, the followingconstraints apply:

    ______________________________________                                        nd.sub.2 < π × 0.4616D, i.e. nd.sub.2 < 1.45D                        md.sub.3 < π × 0.8436D, i.e. md.sub.3 < 2.65D                        d.sub.1 + d.sub.2 < 0.4616D                                                   d.sub.2 + d.sub.3 < 0.382D                                                    ______________________________________                                    

d₃ must be less than 0.1564D to ensure that the outer ring of aperturesis wholly contained within the pipe diameter D. ##EQU2##

The following limits on m thus apply as P is varied:

    ______________________________________                                        P          Minimum m  d.sub.3 /D maximum                                      ______________________________________                                        0.1         3         0.134                                                   0.2         5         0.147                                                   0.3         7         0.152                                                   0.4         9         0.155                                                   0.5        12         0.150                                                   0.6        14         0.152                                                   0.7        16         0.154                                                   0.8        18         0.155                                                   0.9        20         0.155                                                   ______________________________________                                    

note that for P>0.7, md₃ >2.65D.

Optimum performance is likely with a plate porosity in the range 0.5 to0.6.

The limits on m and n for the range of porosity are:

    ______________________________________                                             m        n minimum       n minimum                                       P    minimum  (if d.sub.1 + d.sub.2 < 0.4616D)                                                              (if d.sub.2 + d.sub.3 < 0.382D)                 ______________________________________                                        0.50 12       3               6                                               0.51 12       3               6                                               0.52 12       3               6                                               0.53 12       3               6                                               0.54 12       3               6                                               0.55 13       4               6                                               0.56 13       4               6                                               0.57 13       4               6                                               0.58 13       4               6                                               0.59 13       4               6                                               0.60 14       5               6                                               ______________________________________                                    

Thus the described 1-7-13 plate is just one of a number of plates whichshould give similar downstream flow properties.

Which hole distribution would be most appropriate for a particularinstallation would be a matter of further consideration though therewould seem to be considerable merit in achieving the required downstreamconditions with a plate with a minimum number of holes as this will givea reasonable amount of solid material between adjacent holes in eachring.

I claim:
 1. A flow conditioner comprising an apertured circular plateintended to be placed in a conduit in an orientation substantiallyperpendicular to the axis of the conduit, wherein the apertures arecircular and are arranged in an inner array of apertures adjacent acentral aperture and an outer array of apertures adjacent the innerarray said inner array including n apertures said outer array includingm apertures, the plate having a diameter D, the centers of the aperturesof the inner and outer arrays being located on circles of diameter D₁and D₂ respectively, the central aperture having a diameter d₁, theapertures of the inner and outer arrays having diameters d₂ and d₃respectively, and the array radii and aperture diameters being relatedin accordance with:

    nd.sub.2 D.sub.2 >md.sub.3 D.sub.1,

the center of the central aperture and the centers of the circulararrays coincide with the center of the circular plate, the apertures ineach circular array are equally spaced apart around the center of theplate, all the apertures in any one circular array are of substantiallythe same diameter, and the size and number of apertures in the circulararrays are such that the impedance to flow presented by the plateincreases with the radius on which a given array of apertures isarranged.
 2. A flow conditioner according to claim 1, herein n is equalto seven and m is equal to thirteen.
 3. A flow conditioner according toclaim 2, wherein the inner array has apertures of diameter d₂ equal0.1693 D and a diameter D₁ equal to 0.4616 D, the outer array hasapertures of diameter d₃ equal to 0.1462 D and a diameter D₂ equal to0.8436 D, and the central aperture has a diameter equal to 0.1920 D. 4.A flow conditioner according to claim 1, wherein n is equal to seven andm is equal to eleven or twelve.
 5. A flow conditioner according to claim1, wherein n is equal to six and m is equal to fourteen.
 6. A flowconditioner according to claim 1, wherein n is equal to five and m isequal to twelve.
 7. A flow conditioner according claim 1, wherein thediameter of the central aperture is greater than the diameter of theapertures in the circular arrays, and for any adjacent pair of circulararrays, the apertures in the radially inner array of the pair aregreater diameter than the apertures in the radially outer array of thepair.
 8. A flow conditioner according to claim 1, wherein the open areaof the plate corresponding to the sum of the areas of the apertures isfrom 50 to 60 percent of the total area of the plate.
 9. A flowconditioner according to claim 1, wherein the pressure loss coefficientis at least 2.7.
 10. A flow conditioner according to claim 1, whereinthe plate thickness is at least twelve percent of the plate diameter.11. A flow conditioner according to claim 1, wherein the upstream edgesof each aperture are chamfered.
 12. A flow conditioner according toclaim 11, wherein the chamfered edges subtend an angle of 45° to theplate surface.
 13. A flow conditioner according to claim 11 or 12,wherein the chamfered edges extend to a depth of one sixty fourth of theplate diameter.